1. Field of the Invention
The present invention relates in general to the inspection of glass containers and more particularly to the inspection of wide mouth glass containers for ribbon tear defects.
2. Description of the Prior Art
Glass containers are typically formed by forcing a gas into the interior of a ball of semi-molten glass in a mold. The glass ball expands against the interior surfaces of the mold to form a bottom, side walls and an annular rim defining an opening of the container. During the forming process, various types of defects can also be formed some of which require the container to be rejected. At first, the containers were manually inspected by the human eye but this method proved to be costly, time consuming and inaccurate. Thus, automatic inspection devices were employed in order to reduce the inspection cost and increase the reliability of the inspection process. A typical inspection device is disclosed in U.S. Pat. No. 3,880,750 issued to Richard L. Butler and John W. Juvinall on Apr. 29, 1975 and assigned to Owens-Illinois, Inc. That device inspects the sealing surface of a glass container which is rotated past a light source focused on the sealing surface. A light sensor receives light reflected from the rim to generate an output signal having a magnitude proportional to the amount of received light. A defect will cause a deviation from the d. c. signal level wherein the signal is amplified and filtered to remove the d. c. component. Different types of defects generate different signal levels which are compared with reference signals to identify the various types of defects and generate a defective container signal in response to such a detection.
Another type of defect which generally occurs in the side walls of wide mouth containers is the so-called ribbon tear defect. Such a defect has at least a portion thereof defined by spaced apart edges and has light transmitting properties which differ from those of the side walls. Although it is not fully understood how such defects are formed, one theory is that a portion of the side wall sticks to the interior of the mold as the container is removed. Since most molds are split into two halves along the vertical axis of the container, a pair of seams spaced approximately 180.degree. apart are formed on the side walls. Although the ribbon tear defects often look like the seams, the seam spacing cannot be utilized to distinguish between the tears and the seams because some seams are too thin to generate a reliable signal from a light source and detector system. Therefore, a prior art light source and detector device was made to reject a container only upon a detection which resulted in a large reduction in the d. c. signal level. Although such a device would reject many ribbon tear defects, it would also reject some relatively heavy seams and would not reject some small ribbon tear defects.